1. Field of the Invention
The present invention relates to a solid electrolytic capacitor for which valve metal such as tantalum, niobium or aluminum is used. It also relates to a method of making such a solid electrolytic capacitor.
2. Description of the Related Art
A conventional solid electrolytic capacitor of the above-mentioned type is disclosed in e.g. Japanese Patent Kokoku Publication No. 3(1991)-30977. The conventional capacitor includes a capacitor element, anode and cathode leads connected to the capacitor element, and a resin package enclosing the capacitor element. The anode lead is arranged at one side of the package, while the cathode lead is arranged at the opposite side.
The capacitor element used in such a solid electrolytic capacitor has a prescribed polarity so that the capacitor can function properly only when connected in the correct polarity direction. However, it is sometimes difficult to distinguish between the anode lead and the cathode lead in the conventional capacitor, since each lead is arranged at an end of the package in a similar manner. Accordingly, the conventional capacitor is liable to be mounted on a printed circuit board in the wrong polarity direction.
Further, in the conventional capacitor, the anode lead and the cathode lead are rather widely spaced away from each other since they are disposed at the opposite ends of the package. Unfortunately, this increases the self-induction of the capacitor in a high frequency range.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a solid electrolytic capacitor having an advantageous lead arrangement that can overcome the polarity problem and the self-induction problem accompanying the prior art device. Another object of the present invention is to provide a method of making such a solid electrolytic capacitor.
According to a first aspect of the present invention, there is provided a method of making a solid electrolytic capacitor. The method comprises the steps of: preparing an anode chip made of a valve metal and including a first surface and a second surface opposite to the first surface, the anode chip being provided with an anode bar protruding from the first surface and the second surface of the chip; forming a dielectric layer on the anode chip by anodization; enclosing, by a removable cover, a portion of the anode bar that protrudes from the second surface of the anode chip; forming a solid electrolytic layer by immersing the anode chip in a solid electrolytic layer-forming solution and then baking the anode chip; forming a cathode layer on the solid electrolytic layer; removing the cover; mounting a capacitor element produced by the above-mentioned steps on a metal leadframe, so that portions of the anode bar protruding from the first and the second surfaces of the anode chip are connected to paired anode leads of the leadframe, respectively, and that the cathode layer on the anode chip is connected to a cathode lead of the leadframe that is arranged between the paired anode leads; enclosing the capacitor element by a synthetic resin package in a manner such that the paired anode leads are exposed at opposite ends of the package, and that the cathode lead is exposed at a portion of the package located between the paired anode leads; and cutting the paired anode leads and the cathode lead off the leadframe.
In a solid electrolytic capacitor produced by the above method, two opposite anode leads are located at the opposite ends of the resin package, respectively, and the single cathode is arranged between the paired anode leads. With this symmetrical lead layout, the capacitor can be mounted on a printed circuit board with one correct polarity orientation and also with the reversed orientation. Accordingly, the operator can perform the mounting of the capacitor without worrying about the polarity alignment.
Further, by the middle positioning of the cathode lead between the two anode leads, the distance between the cathode lead and each anode lead is made smaller than is conventionally possible. Accordingly, the self-induction in a high-frequency range can be significantly reduced, whereby the capacitor performance in that range is improved.
According to the above method, the solid electrolytic layer-forming step and the cathode layer-forming step are performed with the protrusion of the anode bar from the anode chip""s second surface enclosed by a cover. Thus, it is possible to prevent the particular protrusion of the anode bar from being formed with a solid electrolytic layer or a cathode layer.
Preferably, the enclosing step by the cover for the anode bar may be performed before the forming step of the dielectric layer on the anode chip. In this manner, no dielectric layer (i.e. insulating layer) is formed on the covered protrusion of the anode bar, whereby a reliable electrical connection can be established between the anode bar""s protrusion and the relevant anode lead.
Preferably, the anode bar may be welded to the paired anode leads, while the cathode layer may be bonded to the cathode lead by a conductive paste. In this manner, the electrical connection between the capacitor element and the leads can be made with ease and at a low cost.
According to a second aspect of the present invention, there is provided a solid electrolytic capacitor that comprises a capacitor element produced by the steps of: preparing an anode chip made of a valve metal and including a first surface and a second surface opposite to the first surface, the anode chip being provided with an anode bar protruding from the first surface and the second surface of the chip; forming a dielectric layer on the anode chip by anodization; enclosing, by a removable cover, a portion of the anode bar that protrudes from the second surface of the anode chip; forming a solid electrolytic layer by immersing the anode chip in a solid electrolytic layer-forming solution and then baking the anode chip; forming a cathode layer on the solid electrolytic layer; and removing the cover. Further, the capacitor of the present invention includes: a first anode lead connected to a portion of the anode bar that protrudes from the first surface of the anode chip; a second anode lead connected to a portion of the anode bar that protrudes from the second surface of the anode chip; a cathode lead connected to the cathode layer formed on the anode chip, the cathode lead being arranged between the paired anode leads; and a synthetic resin package enclosing the capacitor element. The paired anode leads are exposed at opposite ends of the package, while the cathode lead is exposed at a portion of the package between the paired anode leads.
According to a third aspect of the present invention, there is provided a solid electrolytic capacitor comprising: an anode chip including a first surface and a second surface opposite to the first surface; an anode bar including a first portion protruding from the first surface of the chip and a second portion protruding from the second surface of the chip; a cathode layer formed on the anode chip; a first anode lead connected to the first portion of the anode bar; a second anode lead connected to the second portion of the anode bar; a cathode lead connected to the cathode layer; and a resin package enclosing the anode chip and the anode bar, the package including a first end portion and a second end portion opposite to the first end portion. The first anode lead is exposed at the first end portion of the package, the second anode lead is exposed at the second end portion of the package, and the cathode lead is exposed at a location between the first anode lead and the second anode lead.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.